1. Field of the Invention
The present invention relates to a photoconductor for electrophotography (hereinafter also referred to simply as “a photoconductor”), and in particular, to a photoconductor for electrophotography provided with a photosensitive layer that includes organic material on a conductive substrate, and that is used for a printer, a copy machine and the like of an electrophotographic system.
Furthermore, the invention relates to a quinomethane compound, and in detail, to a new quinomethane compound that is useful as an electron transporting material in a photoconductor for electrophotography (hereinafter also referred to as “a photoconductor”), an organic electroluminescence (EL) device and the like.
2. Description of the Related Art
Previously, an inorganic photoconductive material such as selenium or selenium alloy, or a material with an inorganic photoconductive material such as zinc oxide or cadmium sulfide, dispersed in a resin binder, has been used as a photosensitive layer of a photoconductor for electrophotography. In recent years, studies of photoconductors for electrophotography using organic photoconductive materials have progressed to find practical uses for some of the materials with improved sensitivity and durability.
A photoconductor is required to hold surface charges in a dark place, generate charges by receiving light, and transport charges by similarly receiving light. Thus, a single layered photoconductor may provide such functions in one layer, and a layered photoconductor may include a plurality of layers, each layer having a separated function, e.g., a layer contributing mainly to charge generation, and a layer contributing to retention of surface charges in a dark place and transportation of the charges when receiving light.
For example, the Carlson system may be applied to image formation by electrophotography using such photoconductors. The image formation with the system is carried out by charging a photoconductor in a dark place by corona discharge, formation of electrostatic images such as characters and pictures of an original document on the surface of the charged photoconductor, development of the formed electrostatic images with toner, and fixing of the developed toner image onto a support, such as a piece of paper. After the toner image has been transferred, the photoconductor is subjected to erasure of the charge, removal of residual toner, and the like, so that the photoconductor may be used again thereafter.
Organic photoconductors being put to practical use have advantages in flexibility, film formability, low cost, safety and the like, compared with inorganic photoconductors. Furthermore, with a variety of materials, further improvements are made possible with respect to sensitivity, durability, and the like.
Most of the organic conductors are layered organic conductors in which each layer's functions are separate, e.g., have a charge generating layer and a charge transporting layer. In general, the layered organic photoconductor is provided with a charge generating layer and a charge transporting layer, formed in the order recited, on a conductive substrate. The charge generating layer includes a charge generating material, such as a pigment or a dye, and the charge transporting layer includes a charge transporting material, such as hydrazone or triphenylamine. Thus, the organic photoconductor, due to the electron donating property of the charge transporting material, becomes a hole-transport-type of layer that has sensitivity when the surface of the photoconductor is negatively charged. In the negatively charged type of layer, however, compared with a positively charged type of layer, the corona discharge used at charging is unstable. In addition, ozone, nitrogen oxides, and the like are generated, which are adsorbed on the photoconductor surface and may cause physical and chemical degradation. Furthermore, there is a concern about harming the environment. In such respects, a positively charged type photoconductor, having a higher degree of flexibility in service conditions than the negatively charged photoconductor, has a wider scope of application thereof, and is thus more advantageous.
Thus, a method is proposed to use the positively charged type photoconductor and put the photoconductor to practical use, wherein a charge generating material and a charge transporting material are dispersed in a resin binder at the same time to make a photosensitive layer used as a single layer. The single layered photoconductor, however, has a sensitivity that is insufficient for being applied to a high-speed machine, and also necessitates further improvement in repeating characteristics and the like.
In addition, to provide a layered structure of a function separation type that achieves high sensitivity, a method may form a photoconductor by layering a charge generating layer on a charge transporting layer to use the photoconductor as a positively charged type. With the method, however, since the charge generating layer is formed on the surface, there are problems in stability when the layer is repeatedly used, for example, problems due to corona discharge, light irradiation, mechanical wear, and the like. In this case, a protecting layer on the charge generating layer may be provided. Although the protecting layer improves the structure with respect to mechanical wear, degradation in electrical properties, such as sensitivity and the like, are not yet solved.
Furthermore, a method has been proposed for forming a photoconductor by layering a charge transporting layer having an electron transporting property on a charge generating layer.
For example, 2,4,7-trinitro-9-fluorenon and the like may be used as charge transporting materials having an electron transporting property. The materials, however, are carcinogenic, causing a safety problem. In addition to this, cyano compounds and quinone compounds have been proposed in Japanese Patent Publications such as JP-A-1-206349, JP-A-6-59483, JP-A-9-190002, JP-A-9-190003, and the like. However, no electron transport material has been obtained that is sufficient for practical use.
Further, a large number of electron transporting materials and photoconductors for electrophotography using the materials have become known, and are proposed and described in, for example, JP-A-4-360148, Journal of the Society of Electrophotography of Japan, Vol. 30, p. 266 to 273 (1991), JP-A-3-290666, JP-A-5-92936, Proceedings of Pan-Pacific Imaging Conference/Japan Hardcopy 1998, Jul. 15 to 17, 1998 JAHALL, Tokyo, Japan, p. 207 to 210 (1998), JP-A-9-151157, Proceedings of Japan Hardcopy 1997, Jul. 9, 10, 11, 1997, JAHALL (Otemachi, Tokyo), p. 21 to 24 (1997), JP-A-5-279582, JP-A-7-179775, Proceedings of Japan Hardcopy 1992, Jul. 6, 7, 8, 1992, JAHALL (Otemachi, Tokyo), p. 173 to 176 (1992), JP-A-10-73937, JP-A-4-338760, JP-A-1-230054, JP-A-8-278643, JP-A-2001-222122, and the like. Moreover, photoconductors, each using hole transporting materials and electron transporting materials in combination in a single layered photosensitive layer, are noted as being highly sensitive and have begun to be put to practical use. The photoconductors are described in, for example, JP-A-5-150481, JP-A-6-130688, JP-A-9-281728, JP-A-9-281729, JP-A-10-239874, and the like.
Furthermore, there are organic ELs implemented as light-emitting devices using organic photoconductive materials, which devices are expected to be applied to a display, and the like. With respect to the organic ELs, a number of proposals are being presented to improve organic materials, and some of the proposals are being put to practical use.
The simplest structure of the organic EL is a structure in which a light-emitting layer includes a light-emitting material as an organic compound held between the electrodes. A current flows in the electrodes, causing electrons and holes to be injected from the electrodes into the light-emitting layer, by which excitons are formed in the light-emitting layer to bring about a recombination that produces light-emission. Moreover, to efficiently inject electrons and holes from the electrodes into the light-emitting layer, and the like, a structure is also proposed in which functional layers, such as a hole transporting layer, a hole injecting layer, an electron transporting layer, and an electron injecting layer are layered together with a light-emitting layer. Of the layers, for the electron transporting layer and the electron injecting layer, organic compounds which have electron transporting functions are used (See Ohmori, “Recent development of highly efficient organic EL materials,” OYO BUTURI, Vol.70, No.12, p. 1419 to 1425 (2001)).